Gas Cell pressure broadened?

We may be unable use the available DAOF roughing pump to get the gas cell to low pressures necessary to see the true spectral focus. 

My usual procedure for pumping the cell is to put in C2H2, look at 729.5cm-1, 4th order spectra and pump down until the C^13CH2 lines disappear. I did so, and I've been finding disappointing R values for the spectra compared to in CD21. 

For this experiment I turned on the pump and opened the gas cell valve and took exposures at various times after starting. At the start the gas cell had already been pumped down (for 10 minutes at least), but was left untouched for an hour or so while I analyzed data. These are resolutions of fits to lines Q11-Q15 with the 1.9" slit. (Wavenumbers taken from default wavemap from pipeline. I didn't try to touch up the centering.



R (line1) R (line2) R (line3) R (line4) R (line5) time elapsed (min)
67312       71392       67032       73580       67660 0.0 min
72102       75955       70719       78602       71164 4.2 min
71317       76556       70699       77111       70998 6.5 min
71466       76373       70787       77540       70613 10.9 min
72578       75171       71248       78622       71367 17.0 min
73657       75134       71792       80275       72272 23.5 min

I think this suggests we haven't hit the true spectrograph width yet. We've never had to pump down this long ...

If we find another pump and try again that would be good. Regardless, I think we have to live with the results, since my earlier tests showed that we were at best focus on both high-med and medium modes (allowing for the compromise position we adopted in CD21).

UPDATE : 8/3/15-  Yes, the C2H2 lines were suffering pressure broadening.

MMcK used the turbo pump on the gas cell and discovered that the brass adaptor was leaking. 

He replaced it and took a new sequence of Q branch spectra as the cell was pumping down. 

Using file gascell.lab.00200.fits and fitting the lines in the same way as above, I got :

R (line1) R (line2) R (line3) R (line4) R (line5) time elapsed (min)

82205       82327       79504       83283       83683

83107       84126       79668       82536       82248 BEST R***

There were many files to choose from in the sequence, but there's a tradeoff for the quality between line strength, resolution and pumping time. Later I can check to make sure this is the ultimate resolution on slit4, but I think this proves the point that our performance hasn't changed since CD21.

***Update: 8/4/15  I checked the linewidths for all the files from Mark's pumpdown data. Of the ones with good line fits, the best R is file gascell.lab.00202.fits. Therefore, we can expect about R=83000 with slit4.